Film is a very high capacity information storage medium. Under the right storage conditions film has a very slow decay rate which makes it an ideal archive medium. Conventionally film has been used to store picture image information in either monochrome or colour. In this mode the film stores the equivalent of analogue information and possesses an extensive range of densities. Film may also be used in a binary fashion to store digital information.
Film has been scanned for conversion into electrical signals for many years. A prime example of such conversions are the telecine products such as the URSA flying spot, or ADS line array CCD telecines manufactured by Rank Cintel. These telecines convert film data into television signals for use in video and broadcasting applications. These conversions do not extract all the data from the film. Conventional telecines merely extract sufficient data to fit the bandwidth of the television system being used.
Television standards are periodically upgraded as the economics of new technologies permit. Furthermore, the television, video and film industries are continually calling for picture image data to be available in greater detail (resolution) to allow seamless artifact free image processing to be carried out. In this context resolution may appertain to either spatial or temporal resolution or to pixel dynamic level resolution.
Traditionally electronic picture image processing has relied upon bespoke electronic hardware. It has now become practical to use general purpose computer platforms to process picture image data in digital form. To use these computers to carry out seamless artifact free picture image processing on film images requires that these film images are scanned to extract all the information that they contain. This means resolving the detail down to the granular structure of the film stock used, and to resolve the films individual colour density ranges to a sufficiently accurate degree. This is referred to as resolution independent digital film.
To extract all the information from a film frame demands that the image is scanned so as to resolve each colour's data to the point where the inherent media noise (grain) and image dispersion becomes dominant. It has been deduced that with 35 mm negative film this corresponds to approximately 4000 to 6000 picture element sites across its exposed width. A corresponding proportionate number of picture lines are then required across the frame height. This may be 2500 to 4000 lines depending upon the film format and whether square pixel information is required or not.
To obtain the colour density at any pixel requires that light passed is analyzed into its three primary colour components Red (R), Green (G), and Blue (B); and then each of these primary colours is quantified as to its individual relative level. A suitable means for analysing the primary component colours in a film is either to filter the white light into the R,G,B primaries before illuminating the film frame, or to illuminate the film frame with white light and then split the resulting image into its three primary R,G,B components using a colour splitter block. The former technique has the advantage of simplicity and cheapness as only one sensor need be used, but has the disadvantage that a separate scan has to be made for each colour. The latter technique has the disadvantage of expense as a colour splitter and three sensors are required, but has the advantage that only one scan of the film frame is required. A filter wheel may also be used after film illumination but this then puts the filter wheel between the image and image sensor and causes loss of quality of the image.
Sensing light levels can be performed using a variety of technologies. One excellent method is to use Photomultiplier tubes (PMT's). Another technique is to use Avalanche Photo Diodes (APD's), or alternatively to use Charge Coupled Devices (CCD's). All of these devices when correctly driven produce an electrical output which is proportional to the light incident upon them.
The PMT's and APD's require that the light incident upon then is rastered corresponding to scanning the film. This rastering of the light usually involves so called flying spot techniques and is relatively expensive but ideal for real time image scanning.
The CCDs come in two forms--line array and area array. The line array CCD's can sense light in just one line. The area array CCD's can sense light in a plane. Neither CCD types require the light to be rastered. The area array CCD can effectively capture the whole film frame's light variation in one instance whereas the line array CCD can only capture instantaneously one lines worth of the film frames light variation. Thus to use line array CCD sensors requires that the film frame image is moved relative to the sensor in order to capture the whole image. The trade off between using area array versus line array CCDs comes down to one of economics of sensor costs, as the mechanical and electrical cost differences for the two approaches compensate for each other. Needless to say the cost of a line array CCD with 4000 elements is much less expensive than an area array sensor with 1,600,000 elements.
In line array scanners three scans are performed; one scan for each of red, green and blue light. Alternatively three line arrays can be used along with an optical splitter.
There are a number of ways the film image can be moved relative to the line array sensor including:
-- use a flipping mirror PA1 -- use a rotating prism PA1 -- moving the line sensor across the film frame PA1 -- moving the film frame across the line sensor
The use of a flipping mirror or rotating prism suffer from poor resultant image linearity. Moving the line sensor across the film frame gives a resultant image which is subject to optical shading errors which would have to be corrected; this is also a physically bulky element of the system. Moving the film frame across the line sensor is, therefore, preferred.
To obtain mechanical accuracy of the film frame location the Bell & Howell Clapper Gate has been used. This device has been used for the last 50 years in the film printing industry and is readily available from a number of precision mechanics manufacturers around the world (Oxberry, Nielson Hordell) for inclusion in OEM products.
This type of film gate assembly pins the film frame to give mechanical accuracy and has a film lift and advance mechanism to allow the next frame to be brought into the gate aperture and be pinned to the same location. If the film frame were not pinned to the same location then when transferring film frames from a movie the images would be subject to weave and hop due to the variation in interframe location.